2. Detecting Radiation  A Geiger counter is the most familiar tool for detecting radiation.  The probe of this device contains argon gas. When radiation hits the gas it ionizes it, or knocks an electron off.  Ar → Ar + + e -  These electrons falling off creates a weak electric pulse, which makes a speaker click.

3. Half life  Half life is the time it will take for half the material to decay into radiation.  Unstable isotopes have a short half life (3.8 days for Radon-222)  (Carbon-10 has a half life of 19.2 s)  More stable ones have a longer half life (5715 years for Carbon-14)  Stable isotopes have no half life since they do NOT decay. (Carbon-12)

4. More Half life  If two half lives pass…  the material is not gone  you actually have ¼ remaining  half is left after the first half life, then half of that after the second half life.  Not as much radiation is coming out (since there is less mass) but it is still there.

5. Graphing half life mass of isotope number of half lives 0 1 2 3 4

6. Radiodating  The age of materials can be determined if you are capable of comparing the amount of radioactive isotope present now, to the amount of radioactive isotope present at some past date.  Since the half life is a constant rate, you can calculate its age.

7. Finding an age  The amount of C-14 in an object can be measured.  This amount is compared to the amount assumed to be there when it died.  You count the half lives to determine its age.

8. For example  If you measure 15 g of C-14 and you assume you started with 60 g, then the object is…  11,430 years old  60g30g15g (2 half lives)  5715 years x 2 = 11,430 years

9. Types of radiodating  There are different types of radiodating for different scenarios.  For all radiodating techniques, the amount of a radioactive isotope is compared to an amount present in the past.  The half life is used to determine the age

10. Carbon dating  Dates of some materials can determined using carbon-14 or C-14 dating.  Carbon dating can only be used on things once alive.  Carbon dating is ONLY useful for the recent past (50,000 years maximum)  This is done by measuring the number of radioactive C-14 isotopes.

11. How it works  Radiation on this planet causes radioactive isotopes to form.  A known percentage of the carbon dioxide in the air contains the radioactive C-14 isotope.  This carbon dioxide is used to “build” all living things (plants use it for food, animals eat the plants etc.)

12. Continuing  Therefore all living things are made up of a known percentage of C-14.  Once that living thing dies, it stops taking in new C-14 isotopes.  The radioactive isotope begins to decay at a known rate (half life of 5715 years)